This invention relates to an optical modulator using a waveguide formed on a substrate exhibiting the electro-optical Pockels effect.
U.S. Pat. No. 5,278,499 and JP-A-5 273260, disclose an the optical modulator which is used in a device such as a modulator or a device for optically measuring an electromagnetic field intensity, and which and utilizes a substrate of an electro-optic crystal such as LiNbO.sub.3 or the like. In detail, a waveguide of a branched interferometer type is formed by Ti thermal diffusion on the surface of the substrate. The waveguide has an incident end at an end surface of the substrate, extending to the opposite end surface of the substrate with two branched waveguide portions branched at a branched point and exposing their ends at the opposite end surface. Modulating electrodes are formed on the surface of the substrate in the vicinity of the waveguide, and a light reflecting plate is fixedly mounted on the opposite end surface of the substrate by use of an adhering agent of, for example, an ultraviolet curing type.
In the optical modulator, a light beam is incident into the incident end of the waveguide at the end surface of the modulator and propagates in the waveguide. The light beam is split at the branched point into split beams which, in turn, propagate through branched waveguide portions and then is reflected as reflected light beams by the reflecting plate. The reflected light beams propagate backward in the branched waveguide portions and are combined at the branched portion, then outgoing as an output light from the incident end. It will be noted that the incident end is also used as a light outgoing end. The intensity of the output light beam is changed by application of a voltage to the modulating electrode. The intensity change is dependent on a trigonometric function of the voltage applied and has the maximum and the minimum.
The optical modulator has an optical bias point which is determined as an intensity of the output light when the applied voltage is zero. The optical bias point is present at the maximum point of a trigonometric function curve of the output light intensity change if the optical modulator could be formed in a geometrically symmetric form.
The optical modulator is required to have its optical bias point adjusted at a middle point on a linear portion between the maximum and the minimum of the intensity change of the output light. Thus, the light beam is linearly modulated by the voltage applied. On the contrary, when the optical bias point is offset from the middle point towards the maximum or the minimum, the intensity modulation linearity and sensitivity are degraded.
Since it is very difficult to adjust the optical bias point by processes for producing the optical modulator without high production accuracy, the production yield is very low.